Electronic Supplementary Information

FeMoO4 nanorods for efficient ambient electrochemical nitrogen reduction

Jie Wu,a ZhongXu Wang,b Siwei Li,*a Siqi Niu,a Yuanyuan Zhang,a Jing Hu,a Jingxiang Zhao*b and Ping Xu*ac

a MIIT Key Laboratory of Critical Materials Technology for New Energy Conversion and Storage, School of

Chemistry and Chemical Engineering, Harbin Institute of Technology, Harbin 150001, China. Email:

pxu@hit.edu.cn; swli@hit.edu.cnb College of Chemistry and Chemical Engineering, and Key Laboratory of Photonic and Electronic Bandgap

Materials, Ministry of Education, Harbin Normal University, Harbin, 150025, China. Email: xjz_hmily@163.comc Guangdong Provincial Key Laboratory of Energy Materials for Electric Power, Shenzhen 518055, China

Experimental section

Chemicals.

Sodium hypochlorite (NaClO), sodium hydroxide (NaOH), sodium salicylate (C7H5NaO3), sodium

nitroferricyanide (C5FeN6Na2O), ammonium chloride (NH4Cl), hydrazine dihydrochloride

(NH2NH2·2HCl), 4-Dimethylaminobenzaldehyde (p-C9H11NO, PDAB), hydrochloric acid (HCl),

ethanol (C2H5OH), sodiummolybdate dehydrate (Na2MoO4·2H2O), ferrous chloride tetrahydrate

(FeCl2·4H2O), triethylene glycol (TEG), Nafion115, hydrogen peroxide (H2O2), concentrated

sulfuric acid (H2SO4), sodium sulphate (Na2SO4), Nafion (5 wt%) solution.

Preparation of FeMoO4 nanorods

FeMoO4 nanorods were prepared through a solvothermal route. In preparing Solution A, 0.2385

g FeCl2·4H2O was added into 15 mL triethylene glycol (TEG) and then stirred for 0.5 h; In

preparation of Solution B, 0.2903 g Na2MoO4·2H2O was added into 15 mL H2O and then stirred

for 0.5 h. After that, Solution A was added dropwise into Solution B under magnetic stirring for 1

h, during which the mixture solution turned into red brown. Subsequently, the mixture solution

was transferred into a Teflon-lined stainless steel autoclave, and the autoclave was maintained

at 120oC for 24 h in an oven. After cooling to room temperature, the product was collected and

washed with H2O and ethanol repeatedly by centrifugation. Later, the product was dried at 60oC

in a vacuum drier overnight to obtain the FeMoO4 nanorods. The FeMoO4 ink was prepared by

Electronic Supplementary Material (ESI) for ChemComm.This journal is © The Royal Society of Chemistry 2020

ultrasonically dispersing 2 mg of samples into a solution containing 200 µL of H2O, 200 µL of

ethanol and 10 µL of 5 wt% Nafion. And then 15 µL of ink was dropped onto a polished glassy

carbon electrode (GCE) with an area of 0.19625 cm-2 (5mm in diameter), leading to a catalyst

loading of about 0.38 mg cm-2.

Characterizations.

Powder X-ray diffraction (PXRD) data were recorded on a Rigaku D/MAXRC X-ray diffractometer

(45.0 kV, 50.0 mA) by using the Cu target as a X-ray source. The Scanning electron microscopy

(SEM) images and EDS mapping were collected on a Quanta 200 S (FEI). The Transmission

electron microscopy (TEM) images were conducted on a Tecnai F20 operating at an accelerating

voltage of 200 kV. The X-ray photoelectron spectra (XPS) were collected on a PHI 5700 ESCA

system using Al Kα radiation as a source. The Raman spectra were obtained on a Renishaw inVia

microRaman spectroscopy system (wavelength: 532 nm).

Electrochemical measurements.

Nafion 115 membrane was pretreated by H2O, 5% H2O2 and 0.5 M H2SO4 at 80oC for 1 h before

electrochemical tests. The electrochemical measurements were conducted on a CHI 830D

electrochemical analyzer (CHI Instruments, Inc.) in a standard three-electrode cell, using Ag/AgCl

(saturated KCl solution) as the reference electrode, a GCE (5mm in diameter) loaded with

catalysts as the working electrode and a graphite rod as the counter electrode. Before NRR, N2 or

Ar were purged into 0.1 M Na2SO4 electrolyte for 0.5 h. The NRR tests were carried out for 2 h

under N2- and Ar-saturated Na2SO4 electrolyte at room temperature. 4 mL of electrolyte after

chronoamperometry lasted for 2h was collected to determine the concrete NH3 concentration,

using the indophenol blue method.

Determination of NH3.

Coloring solution: sodium salicylate (0.4 M) and sodium hydroxide (0.32 M).

Oxidation solution: sodium hypochlorite (ρCl = 4-4.9) and sodium hydroxide (0.75 M).

Catalyst solution: 0.1 g Na2[Fe(CN)5NO]·2H2O diluted to 10 mL with deionized water

4mL of solution was collected from electrolyte. And then 50 µL of oxidation solution, 500 µL of

coloring solution and 50 µL of catalyst solution were added, respectively. After coloring for 2 h at

room temperature, the absorbance at λ=690 nm of the UV-Vis spectrum was collected.1

Afterwards, the NH3 concentration was calculated through the concentration-absorbance curve

which was line-fitted from a series of standard ammonia solutions of different concentrations.

The obtained linear curve of NH3 is Y=0.822X + 0.0511, R2=0.999.

Determination of N2H4.

Coloring solution: 4 g PDAB, 20mL HCl and 200mL ethanol were added together, followed by

stirring for few minutes to form a clear solution. 4 mL of solution was collected from electrolyte,

followed by adding 4 mL of the coloring solution. After keeping for 1 h at room temperature, the

absorbance at λ=455 nm of the UV-Vis spectrum was collected. And then, the concentration N2H4

was calculated through the concentration-absorbance curve which was line-fitted from a series

of standard hydrazine hydrate solutions of different concentrations. The obtained linear curve of

N2H4.H2O is Y=1.071X + 0.025, R2=0.999.

Calculations of NH3 formation rate and Faradic efficiency (FE).

NH3 yield rate = (c V) / (t m cat.)

where c is the concentration of NH3, V is the volume of the electrolyte, t is the reaction time, and

m cat. is catalyst weight.

FE = 3F c V / (17 Q)

where F is the Faraday constant, c is the concentration of NH3, V is the volume of the electrolyte,

and Q is total charge consumed for the electrodes.

Computational Methods.

Spin-polarized DFT computations were performed by employing the Vienna Ab Initio Simulation

Package (VASP).2 The exchange–correlation energy was represented by the Perdew–Burke–

Ernzerhof (PBE) functional within the generalized gradient approximation (GGA) and the

electron–ion interactions were described by the projector augmented wave (PAW) method.3, 4

Consistent with our experimental results, the (110) surface of cubic FeMoO4 was constructed to

explore the possible NRR activity. Dipole correction was employed to correct potential spurious

terms arising from the asymmetry of the slabs.5 The DFT-D3 method was used to include the van

der Waals interactions.6 A (3 × 3 × 1) Monkhorst-Pack k-point mesh was used to sample the

Brillouin zone, and the plane wave cut-off energy was set to be 480 eV in all computations.

Geometry optimizations were performed with a convergence threshold of 10-3 eV in energy and

0.05 eV/Å for the force. A vacuum of 20 Å along the z-direction was used, which was large

enough to minimize the interactions between periodic images.

The Gibbs reaction free energy change (△G) of each elementary was calculated based on a

computational hydrogen electrode (CHE) model,7 in which the chemical potential of the

proton/electron (H+ + e-) pair in aqueous solution is equal to one-half of the chemical potential

of a hydrogen molecule. The adsorption energies (Eads) of adsorbate was determined by Eads =

Etotal - Eslab - Eadsorbate, where Etotal, Eslab, and Eadsorbate are the DFT total energy for the adsorbate on

the slab system, the clean slab, and the adsorbate itself, respectively. The change in Gibbs free

energy (ΔG) was obtained using △G = △E +△ZPE - T△S, where △E is the reaction energy

difference of the reactants and products directly determined by the DFT computations and △ZPE

and △S are the changes of zero point energies and entropies between the adsorbed species. T

was set at 298.15 K in this work, and the entropies of the free molecules were taken from the

NIST database.

Supplementary figures and tables

Fig. S1 O 1s XPS spectrum of FeMoO4 Nanorods.

Fig. S2 The calibration LSV curve of the reference electrode (Ag/AgCl) in 0.1 M Na2SO4 at room temperature.

Fig. S3 (a) The UV-Vis spectra of various concentrations of NH4+, using indophenol blue method;

(b) The standard curve used for assessment of NH3 concentrations.

Fig. S4 (a) The UV-Vis spectra of various concentrations of N2H4, using Watt and Chrisp method; (b)The standard curve used for assessment of N2H4 concentrations.

Fig. S5 The UV-Vis spectra of electrolyte at various applied potentials for 2h.

Fig. S6 The UV-Vis spectra of electrolyte at -0.6 V for 2h and the blank electrolyte, using the Watt and Chrisp analytical method.

Fig. S7 Stability of FeMoO4 nanorods as an NRR catalyst at -0.6 V in neutral media.

Fig. S8 The UV-Vis spectra of electrolyte at -0.6V versus RHE for 2h, where FeMoO4-1,FeMoO4-2

and FeMoO4-3 are the products of parallel synthetic experiments.

Fig. S9 The UV-Vis spectra of electrolytes at -0.6 V for 2h using GCE and FeMoO4/GCE as the working electrode, and of the blank electrolyte.

Fig. S10 The UV-Vis spectra of electrolyte at open circuit condition for 2h using bare GCE and of the blank electrolyte.

Fig. S11 The UV-Vis spectra of electrolyte at -0.6 V for 2h under Ar-saturated atmosphere using FeMoO4/GCE and of the blank electrolyte.

Fig. S12 The adsorption of N2 molecule on the (a) Mo and (b) Fe sites and the corresponding adsorption energies and key parameters.

Fig. S13 The spin densities of FeMoO4 with the isovalue of 0.05 e/Å3. Fe site exhibits much larger spin densities than Mo site (1.54 μB on per Fe site and 0.02 μB on per Mo site)

Table S1 The NRR performance of FeMoO4, comparing with other reported catalysts

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